The invention relates to wireless communication systems. In particular, the invention relates to dynamically recognizing an empty general paging message received by a remote unit in a wireless communication system.
A wireless communication system may comprise multiple remote units and multiple base stations. FIG. 1 exemplifies an embodiment of a terrestrial wireless communication system with three remote units 10A, 10B and 10C and two base stations 12. In FIG. 1, the three remote units are shown as a mobile telephone unit installed in a car 10A, a portable computer remote 10B, and a fixed location unit 10C such as might be found in a wireless local loop or meter reading system. Remote units may be any type of communication unit such as, for example, hand-held personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link 14 from the base station 12 to the remote units 10 and a reverse link 16 from the remote units 10 to the base stations 12.
Communication between remote units and base stations, over the wireless channel, can be accomplished using one of a variety of multiple access techniques which facilitate a large number of users in a limited frequency spectrum. These multiple access techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). An industry standard for CDMA is set forth in the TIA/EIA Interim Standard entitled xe2x80x9cMobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, TIA/EIA/IS-95, and its progeny (collectively referred to here as IS-95), the contents of which are incorporated by reference herein in their entirety. Additional information concerning a CDMA communication system is disclosed in U.S. Pat. No. 4,901,307, entitled SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS, (the ""307 patent) assigned to the assignee of the present invention and incorporated in its entirety herein by reference.
In the ""307 patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through base stations using CDMA spread spectrum communication signals. The CDMA modulation techniques disclosed in the ""307 patent offer many advantages over other modulation techniques used in wireless communication systems such as TDMA and FDMA. For example, CDMA permits the frequency spectrum to be reused multiple times, thereby permitting an increase in system user capacity. Additionally, use of CDMA techniques permits the special problems of the terrestrial channel to be overcome by mitigation of the adverse effects of multipath, e.g. fading, while also exploiting the advantages thereof.
In a wireless communication system, a signal may travel several distinct propagation paths as it propagates between base stations and remote units. The multipath signal generated by the characteristics of the wireless channel presents a challenge to the communication system. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.
In the wireless channel, the multipath is created by reflection of the signal from obstacles in the environment such as, for example, buildings, trees, cars, and people. Accordingly, the wireless channel is generally a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses changes in time delay, attenuation, and phase as a function of the time that the ideal impulse is transmitted.
The multipath characteristics of a channel can affect the signal received by the remote unit and result in, among other things, fading of the signal. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors add destructively, yielding a received signal that is smaller in amplitude than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of xcex4 with a phase shift of "THgr" radians, and the second path has an attenuation factor of X dB, a time delay of xcex4 with a phase shift of "THgr"+xcfx80 radians, no signal is received at the output of the channel because the two signals, being equal amplitude and opposite phase, cancel each other. Thus, fading may have a severe negative effect on the performance of a wireless communication system.
A CDMA communication system is optimized for operation in a multipath environment. For example, the forward link and reverse link signals are modulated with a high frequency pseudonoise (PN) sequence. The PN modulation allows the many different multipath instances of the same signal to be separately received through the use of a xe2x80x9crakexe2x80x9d receiver design. In a rake receiver, each element within a set of demodulation elements can be assigned to an individual multipath instance of a signal. The demodulated outputs of the demodulation elements are then combined to generate a combined signal. Thus, all of the multipath signal instances must fade together before the combined signal experiences a deep fade.
In a communication system based on the industry standard for CDMA, IS-95, each of the multiple base stations transmits a pilot signal having a common PN sequence. Each base station transmits the pilot signal offset in time from neighboring base stations so that the signals can be distinguished from one another at the remote unit. At any given time, the remote unit may receive a variety of pilot signals from multiple base stations. Using a copy of the PN sequence produced by a local PN generator, the entire PN space can be searched by the remote unit. Using the search results, a controller in the remote unit distinguishes pilot signals from multiple base stations based on the time offset.
The controller is used to assign demodulation elements to the available multipath signal instances. A search engine is used to provide data to the controller concerning the multipath components of the received signal. The search engine measures the arrival time and amplitude of the multipath components of a pilot signal transmitted by the base stations. The effect of the multipath environment on the pilot signal and the data signal transmitted by a common base station is very similar because the signals travel through the same channel at the same time. Therefore, determining the multipath environment""s affect on the pilot signal allows the controller to assign demodulation elements to the data channel multipath signal instances.
The search engine determines the multipath components of the pilot signals of base stations in the proximity of the remote unit by searching through a sequence of potential PN offsets and measuring the energy of the pilot signal received at each of the potential PN offsets. The controller evaluates the energy associated with a potential offset, and, if it exceeds a threshold, assigns a signal demodulation element to that offset. A method of and an apparatus for demodulation element assignment based on searcher energy levels is disclosed in U.S. Pat. No. 5,490,165 entitled DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS, (the ""165 patent) assigned to the assignee of the present invention.
FIG. 2 shows an exemplifying set of multipath signal instances of a single pilot signal from a base station arriving at a remote unit. The vertical axis represents the power received in decibels (dB). The horizontal axis represents the delay in the arrival time of a signal instance due to multipath delays. The axis (not shown) going into the page represents a segment of time. Each signal spike in the common plane of the page has arrived at the remote unit at a common time but has been transmitted by the base station at a different time. Each signal spike 22-27 has traveled a different path and therefore exhibits a different time delay, a different amplitude, and a different phase response. The six different signal spikes 22-27 are representative of a severe multipath environment. A typical urban environment produces fewer usable paths. The noise floor of the system is represented by the peaks and dips having lower energy levels. The task of the search engine is to identify the delay, as measured by the horizontal axis, and amplitude, as measured by the vertical axis, of the signal spikes 22-27 for potential demodulation element assignment.
Note, as shown in FIG. 2, each of the multipath peaks varies in amplitude as a function of time as shown by the uneven ridge of each multipath peak. In the limited time shown, there are no major changes in the multipath peaks. Over a more extended time range, multipath peaks disappear and new paths are created as time progresses. Multipath peaks are likely to merge together or blur into a wide peak over time.
Typically, the operation of the search engine is overseen by a controller. The controller commands the search engine to step through a set of offsets, called a search window, that is likely to contain one or more multipath signal peaks suitable for assignment to a demodulation element. For each offset, the search engine reports the energy it found offset back to the controller. Demodulation elements may then be assigned by the controller to the paths identified by the search engine (i.e. the timing reference of their PN generators is aligned with the timing of the identified path). Once a demodulation element has locked onto the signal, it then tracks that path on its own without controller supervision, until the path fades away or until the demodulation element is assigned to another path by the controller.
As noted above, each base station in a given geographical area is assigned a sequence offset of a common PN pilot sequence. For example, according to IS-95, a PN sequence having 215 chips (i.e., one bit in the PN sequence) and repeating every 26.66 milliseconds (ms) is transmitted by each base station in the system at one of 512 PN sequence offsets as a pilot signal. According to IS-95 operation, the base stations continually transmit the pilot signal which can be used by the remote unit to identify the base station as well as other functions such as, for example, determining the multipath environment the remote unit is operating in and synchronization of remote unit timing to the base station timing.
During initial power on, or any other situation when the remote unit has lost a pilot signal such as when performing a hard hand-off to a different operating frequency, the remote unit evaluates all possible PN offsets of the pilot PN sequence. Typically, a search engine measures the pilot signal strength at all possible PN offsets, proceeding at a measurement rate that produces an accurate measure of the pilot signal present at the corresponding offset. Proceeding in this manner, the search engine determines the PN offset of base stations which are geographically near the remote unit. Searching each PN offset in this manner can take anywhere from hundreds of milliseconds to a few seconds depending on the channel conditions during acquisition. This amount of time for the remote unit to reacquire a pilot signal is detrimental to the remote unit operation and may be annoying to the user of the remote unit.
FIG. 3 shows an extended portion of PN space on the horizontal axis. The three groups of peaks 30, 32, 34 represent transmissions from three different base stations. As shown, the signal from each base station signal experiences a different multipath environment. Also, each base station has a different PN offset from the PN reference 36. Thus, the controller may select a set of PN offsets corresponding to search windows for any of the identified base stations. This allows the remote unit to simultaneously demodulate signals from multiple base stations by assigning demodulation elements appropriately.
In a typical CDMA communication system, remote units sporadically establish bi-directional communications with a base station. For example, a cellular telephone remains idle for significant periods of time when no call is in process. However, to ensure that any message directed to a remote unit is received, the remote unit continuously monitors the communication channel, even while it is idle. For example, while idle, the remote unit monitors the forward link channel from the base station to detect incoming calls. During such idle periods, the remote unit continues to consume power to sustain the elements necessary to monitor for signals from the base stations. Many remote units are portable and are powered by an internal battery. For example, personal communication system (PCS) handsets are almost exclusively battery-powered. The consumption of battery resources by the remote unit in idle mode decreases the battery resources available to the remote unit when a call is placed or received. Therefore, it is desirable to minimize power consumption in a remote unit in the idle state and thereby increase battery life.
One means of reducing remote unit power consumption in a communication system is disclosed in U.S. Pat. No. 5,392,287, entitled APPARATUS AND METHOD FOR REDUCING POWER CONSUMPTION IN A MOBILE COMMUNICATION RECEIVER (the ""287 patent), assigned to the assignee of the present invention and hereby incorporated in its entirety herein by reference. In the ""287 patent, a technique for reducing power consumption in a remote unit operating in an idle mode (i.e. a remote unit which is not engaged in bi-directional communication with a base station) is disclosed. In idle, each remote unit periodically enters an xe2x80x9cactivexe2x80x9d state during which it prepares to and receives messages on a forward link communication channel. In the time period between successive active states, the remote unit enters an xe2x80x9cinactivexe2x80x9d state. During the remote unit""s inactive state, the base station does not send any messages to that remote unit, although it may send messages to other remote units in the system that are in the active state.
As disclosed in the ""287 patent, a base station broadcasts messages which are received by all remote units within the base station coverage area on a xe2x80x9cpaging channel.xe2x80x9d All idle remote units within the base station coverage area monitor the paging channel. The paging channel is divided in the time dimension into a continuous stream of xe2x80x9cslots.xe2x80x9d Each remote unit operating in slotted mode monitors only specific slots which have been assigned to it as assigned slots. The paging channel continually transmits messages in numbered slots, repeating the slot sequence such as, for example, every 640 slots. When a remote unit enters the coverage area of a base station or if a remote unit is initially powered on, it communicates its presence to a preferred base station. Typically, the preferred base station is the base station which has the strongest pilot signal as measured by the remote unit.
The preferred base station, along with a plurality of geographically near neighboring base stations, assign a slot, or a plurality of slots, within their respective paging channels, for the remote unit to monitor. The base station uses the slots in the paging channel to transmit control information to a remote unit, if necessary. The remote unit may also monitor a timing signal from the preferred base station allowing the remote unit to align, in the time dimension, to the base station slot timing. By aligning in the time dimension to the preferred base station slot timing, the remote unit can determine when a paging channel slot sequence begins. Thus, knowing when the paging channel slot sequence begins, which slots are assigned for it to monitor, the total number of slots in the repetitive paging channel sequence of slots, and the period of each slot, the remote unit is able to determine when its assigned slots occur.
Generally, the remote unit is in the inactive state while the base station is transmitting on the paging channel in slots which are not within the remote unit""s assigned set. While in the inactive state, the remote unit does not monitor timing signals transmitted by the base station, maintaining slot timing using an internal clock source. Additionally, while in the inactive state the remote unit may remove power from selected circuitry such as, for example, circuits which monitor pilot signals transmitted by base stations to detect changes in the wireless channel including the search engine. Using its internal timing, the remote unit transits to its active state a short period of time before the next occurrence of an assigned slot.
When transitioning to the active state, the remote unit applies power to circuitry that monitors the wireless channel, including the search engine. The search engine is used to reacquire the preferred base station""s pilot signal and to detect changes in the wireless channel which may have occurred due to the movement of the remote unit or to the movement of objects within the coverage area of the base station. In addition to reaquiring the pilot signal, the remote unit may perform any other actions or initializations in preparation of receiving a message at the beginning of its assigned slot.
When the remote unit enters the active state, it may receive messages in its assigned slots in the paging channel and respond to commands from the base station. For example, the remote unit may be commanded to activate a xe2x80x9ctrafficxe2x80x9d channel to establish a bi-directional communication link for conducting subsequent voice communication in response to an incoming call. If there is no message from the base station, or no command requesting the remote unit to remain active, at the end of the assigned slot the remote unit returns to the inactive state. In addition, the remote unit returns to the inactive state immediately if commanded to do so by the base station in the form of, for example, a sleep command.
During its assigned slot, the remote unit""s search engine measures the pilot signal strength of the preferred base station as well as the pilot signal strengths of neighboring base stations. If the remote unit relocates from the coverage area of one base station to another neighboring base station""s coverage area, the remote unit needs to xe2x80x9chand-offxe2x80x9d to the neighboring base station. A hand-off occurs when the transmitted pilot signal strength of a neighbor base station becomes sufficiently stronger than the preferred base station. When this occurs, the neighboring base station is assigned as the preferred base station. Following a hand-off, in the next active state, the remote unit monitors the paging channel of the new preferred base station to receive messages and commands.
In addition to providing data for determining when a hand-off should occur, searches of the preferred base station""s pilot signal allow the remote unit to make adjustments to compensate for changes in the multipath environment. For example, if one of the multipath signal instances weakens to the point that it is unusable, the remote unit may reassign demodulation elements accordingly.
Knowing the nominal PN offset of the preferred base station as well as a neighboring set of base stations, the controller typically passes a set of search parameters to the search engine specifying PN offsets at which multipath signal instances of pilot signals are likely to be found. At the completion of the search, the search engine passes the search results to the controller. The controller analyzes the search results and selects a set of search parameters for the next search. Following selection of the new search parameters, the controller passes the parameters to the search engine and the search process is repeated. This process is repeated until the remote unit once again enters the inactive, idle state.
During the assigned slot, the base station sends a general paging message as well as other message types. The general paging message carries information such as whether all messages directed to remote units operating in the slotted mode and active in the current slot have been sent by the end of this general paging message, whether all broadcast messages have been sent by the end of the current general paging message, the class of remote units which are being addressed, service option requests directed to a specific remote unit and other messages such as, for example, those described in IS-95.
When no more remote unit-specific or broadcast messages are available for the assigned slot time, the base station sends an empty general paging message. Upon receipt of the empty general paging message, the remote units simply return to inactive state.
FIG. 4 is block diagram showing a portion of a prior art remote unit used to receive the general paging message. A receiver 40 receives wireless link signals including the general paging message. The receiver 40 provides for reception and down-conversion of the wireless link signal and also provides despreading in a CDMA environment as well as other demodulation functions. The receiver 40 provides a series of digital bit values at its output.
According to well-known wireless link protocols such as IS-95, before data is transmitted over the wireless link, it is divided into a series of blocks. The blocks are reordered in time such that the block order is non-time sequential as transmitted over the wireless link period. This method of transmitting the blocks is referred to as interleaving, and the process of reordering the blocks is referred to as deinterleaving. Interleaving helps mitigate the deleterious effects of fading on the wireless channel by reducing the probability that a time-sequential set of blocks will be lost during a fade. Instead, when a series of blocks are lost due to a fade, they are non-time sequential after the deinterleaving process and, thus, are more easily corrected by the decoding process. A deinterleaver 42 performs the deinterleaving function. The deinterleaver 42 receives samples from the receiver 40 and accumulates a series of block data. When an entire set of blocks has been received, the deinterleaver 42 reorders the blocks in time-sequential order and outputs them to a decoder 44.
The decoder 44 can be a convolutional decoder operating at rate one-half (R=xc2xd) with a constraint length of nine (K=9). One common form of convolutional decoders is the Virterbi decoder. A Virterbi decoder creates soft decision data based upon groups of data. The entire group of data must be presented to the decoder before the decoder presents a soft decision output. The soft decision data is collected in a buffer within the decoder 44 for processing.
In general, the general paging message is comprised of several groups of decoder data. When the decoder buffer contains sufficient data, the data is passed to a message parser 46. The message parser 46 performs such functions as collection of bits in the message, computing and verifying any cyclic redundancy code (CRC) or other error checking code, translation of the message into an internal format, copying the transformed message into a buffer, and placement of the transformed message onto a queue for the proper protocol task. The message is evaluated field-by-field. In general, the processes of the decoder 44 and the message parser 46 are controlled by a controller 48.
There is a need in the art to minimize the amount of time required by the remote unit to parse a general paging message before determining if the remote unit is to enter the inactive state. The processes carried out by a message parser are extensive and require a significant amount of time. Until the message parser is freed from the general paging message, the remote unit remains active. It is only after receipt of a sleep command from the base station or the receipt of an empty general paging message that the remote unit returns to the inactive state.
In general, the general paging message spans across several sets of deinterleaved blocks of data. Thus, in order to process the entire general paging message, the message parser must wait until all of the blocks which comprise the general paging message have been received. For this reason, previously, little advantage was obtained by increasing the speed at which the decoder operates. Even if the decoder was operated at a high frequency, the message parsing task must wait for multiple deinterleaving processes to be executed. Thus, increasing the rate at which the decoder operates merely increases the amount of time that the data at the output of the decoder is buffered before message processing begins.
The invention is directed to methods of and an apparatus for recognizing an empty general paging message in a wireless communication system. In an aspect of the present invention, the invention can be used for reducing the average power consumption in a remote unit. In a slotted mode communication system, a remote unit will alternate between active and inactive states. When the remote unit is in its active state it monitors the wireless channel and receives messages from a base station. At the completion of its active state, either when commanded by the base station or reaching the end of its assigned slot, the remote unit enters an inactive state. During the inactive state the remote unit reduces power consumption by removing power from selected circuitry, such as, for example, circuitry used to monitor the wireless channel.
A method of processing base station signals by a remote unit according to one aspect of the present invention includes receiving raw data samples of a message from a base station; decoding the raw data samples to obtain decoded data samples; comparing the decoded data samples with stored reference samples before the decoded data samples are parsed; and determining, based on the comparing, contents of the message.
In another aspect of the invention, the message may be received in an interleaved form and may be deinterleaved after the receiving.
In a further aspect of the invention, the method may further comprise transitioning the remote unit to a sleep mode when the determining determines the message is empty.
In another aspect of the invention, a remote unit includes a receiver configured to receive paging messages as raw data samples; a decoder configured to obtain decoded data samples from the raw data samples; a memory configured to store data samples; and a comparator configured to compare the decoded data samples with the data samples in the memory to determine contents of the paging messages.
In yet another aspect of the invention, the receiver may be configured to receive the paging message in an interleaved form and may be configured to deinterleave said paging message.
In a further aspect of the invention, the remote unit, when no stored reference samples exist in memory, may parse the paging message and may store at least a portion of the paging message if the paging message is empty.